The present invention relates to a gas sensing device, and more particularly concerns a gas sensing device whose electrical properties vary with the type of gas sorbed therein.
The literature contains descriptions of many types of devices and methods which are capable of detecting gases including chemical reactions which give color changes or pH changes, instrumental methods including mass spectrometers and infrared analyzers, electrochemical techniques which utilize the properties of dissolved gases, solid state devices which detect changes in thermal and electrical conductivities of different gases, thin film solid state devices such as thin oxide films, organic semiconductor films which exhibit electrical conductivity changes as a function of ambient gas composition, and chemical field effect transistors.
A wide variety of different means have been used to detect the presence of particular gases over time, these means ranging from the organic (such as the use of a canary in a coal mine) to the complex semiconductor or solid state devices described in the ACS Symposium Serie 309 "Fundamentals and Applications of Chemical Sensors" (1986 American Chemical Society) and in patents such as U.S. Pat. No. 4,058,368 (disclosing a hydrogen detector MIS device comprising a semiconductor substrate, a metal electrode of palladium, nickel, platinum or palladium alloy, and an intervening insulator situated between the semiconductor substrate and the metal electrode). Devices of silicon and palladium Schottky layer have been used as gas detectors, the dark current or capacitance being measured by impressing an external voltage across the diode,
In one important aspect the earlier organic gas sensor--namely, the canary--provided an advantage not found in the more sophisticated devices presently in favor. The canary did not require an outside power supply. By way of contrast, the prior art gas sensing devices have been subjected to external power or internal power supply (e.g., battery) failures which could lead to deactivation of the device even in situations where a momentary deactivation could be fatal. In those applications where the cost is warranted, fail safe devices are utilized to bring the user's attention the failure of the power supply, but the device remains inoperative for its intended purpose of detecting gas.
Accordingly, it is an object of the present invention to provide a self-powered gas detecting device which requires neither an internal electrical storage system nor connection to an external power supply.
Another object is to provide such a gas sensing device which, when exposed to light, generates its own photovoltage or photocurrent, such photovoltage or photocurrent changing as a function of the ambient gas composition.
Still another object is to provide such a device which can discriminate between particular gases.
A further object is to provide such device which is inexpensive to construct and maintain and easy and economical to operate.